Many electronic packages require hermetically sealed connections from the inside of the package to the outside, such as electrical pins hermetically sealed through a hole in the package wall. The fiber optic industry, in particular, requires optical fibers passing through the package wall to the outside. Historically, the fibers were soldered into a feed-through tube which was in turn soldered to an opening in the wall, creating a hermetic feed-through of the optical fiber.
The soldering operation for a fiber optic assembly is relatively expensive, requiring metallization of the optical fiber for solder wetting. More recently, tiny glass preforms were substituted for the solder in the end of the feed-through tube. This eliminated the requirement for metallization and the associated atmosphere and cleaning requirements of soldering, thereby reducing the manufacturing costs of a fiber optic assembly significantly. The glass preforms are made by mixing glass powder with a binder which gave the preform green strength after pressing. After pressing, the preforms are fused to give green strength for shipping and handling. The preforms, shaped like tiny donuts with an inside diameter as small as 0.010″, are then loaded by hand over the optical fibers and rested on the end of the feed-through tube. The resulting assembly of tube, fiber, and preform is then heated to flow the glass and seal the fiber into the tube.
Although the glass preforms were a major improvement over solder, they also had some disadvantages. The tiny preforms are relatively expensive to manufacture with consistently high quality. Sometimes minor fluctuations in the glass preform manufacturing process caused significant yield loss in a fiber optic assembly line. Once manufactured, preforms have to be stored in moisture free atmosphere. Failure of placing them in dry atmosphere would cause them to hydrolyze and become fragile. A more significant drawback was that there was no easy way to automate the loading of the tiny preforms over the optical fibers fed through the feed-through tube. In this case, the feed-through tube was a tiny cylindrical gold plated Kovar metal alloy tube with a less than 10 mil hole bored through the length. The loading of the glass preforms manually was the bottleneck in the production line.
A variety of techniques have been suggested in the past for dealing with similar problems.
Sinclair et al., in U.S. Pat. No. 4,695,125, teach a passive fiber optic device and optical fiber connected to it that are packaged in a solid block of a bismuth-containing fusible alloy. The alloy exhibits appreciable expansion on solidification, negligible dimensional change after solidification, and a low thermal expansion coefficient. Coupled with its low melting point and glass wetting ability, the alloy is uniquely adapted for forming a hermetic seal with glass. By molding as a solid block, the packaging operation is rendered simple and rapid.
Knecht et al., in U.S. Pat. No. 4,994,134, disclose a method of making a two-layered (ceramic-glass) ferrule having a high degree of concentricity employing the steps of: (a) providing a tubular fixture; (b) inserting a ferrule fixture into a portion of the tubular fixture, the ferrule fixture having continuous and non-continuous channels, the concentricity of the continuous channel varying no more than two microns; (c) inserting a ceramic ferrule tubular member into the tubular fixture; (d) inserting a glass ferrule tubular member into the ceramic ferrule tubular member and into the non-continuous channel of the ferrule fixture; (e) inserting an alignment member, having a length greater than the glass ferrule tubular member and a diameter that matches the internal diameter of the glass ferrule tubular member and the continuous channel of said ferrule fixture within one micron, into the continuous channel of the ferrule fixture and into the glass ferrule tubular member, (f) bonding the ceramic ferrule tubular member to the glass ferrule tubular member to form a composite; and (g) removing the alignment member and glass-ceramic composite from the tubular fixture.
Kramer, in U.S. Pat. No. 5,143,531, teaches a glass-to-glass hermetic sealing technique which can be used to splice lengths of glass fibers together. A solid glass preform is inserted into the cavity of a metal component which is then heated to melt the glass. An end of an optical fiber is then advanced into the molten glass and the entire structure cooled to solidify the glass in sealing engagement with the optical fiber end and the metal cavity.
Kramer, in U.S. Pat. No. 5,337,387, teaches hermetic fiber optic-to-metal components and a method for making hermetic fiber optic-to-metal components by assembling and fixturing elements comprising a metal shell, a glass preform, and a metal-coated fiber optic into desired relative positions and then sealing said fixtured elements preferably using a continuous heating process. The resultant hermetic fiber optic-to-metal components exhibit high hermeticity and durability despite the large differences in thermal coefficients of expansion among the various elements.
Bookbinder et al., in U.S. Pat. No. 5,475,784, teach a method of encapsulating an optical component, the component comprising at least partially uncoated organic material, the method comprising placing molten metal around the optical component and solidifying the metal. The invention includes encapsulating a segment or element of an optical component, such as an optical junction or a surface. An encapsulated optical component and optical components having sealed or encapsulated elements are also provided.
Kramer, in U.S. Pat. No. 5,568,585, teaches a method for manufacturing low-temperature hermetically sealed optical fiber components. The method comprises the steps of: inserting an optical fiber into a housing, the optical fiber having a glass core, a glass cladding and a protective buffer layer disposed around the core and cladding; heating the housing to a predetermined temperature, the predetermined temperature being below a melting point for the protective buffer layer and above a melting point of a solder, placing the solder in communication with the heated housing to allow the solder to form an eutectic and thereby fill a gap between the interior of the housing and the optical fiber, and cooling the housing to allow the solder to form a hermetic compression seal between the housing and the optical fiber.
Tanabe et al., in U.S. Pat. No. 5,613,031, to teach a package member which has fitted in a through hole thereof a pipe member with an optical fiber inserted therein, the optical fiber including a jacketed part thereof fitted in and soldered to an inside region of the pipe member, with a solder filled there between, and a stripped part thereof fitted in and fixed to another inside region of the pipe member, with an adhesive filled there between, while the pipe member is fixed to the package member by a welding along a circumference of the through hole, without using a fixing ring, permitting a relatively high hermeticity to be achieved in a facilitated fabrication process.
DeVore et al., in U.S. Pat. No. 5,658,364, teach a method of making fiber optic-to-metal connection seals. The optical fiber and a preform made of a sealing material are inserted into a metal cup. The metal is then heated to a temperature which melts the sealing material to form a hermetic seal between the cup and the fiber optic. The hermetic sealing material is selected from glass, glass-ceramic or braze and the metal is selected from stainless steel, a metal alloy or a high-strength superalloy.
Beranek et al., in U.S. Pat. No. 5,692,086, teach an optoelectronic package which includes an optical fiber cable assembly and feed-through assembly which provide high performance and high reliability optical fiber alignment, locking and sealing. An optical fiber is fed through a nose tube into the package. The fiber is selectively metallized at its end. A solder lock joint on a substrate on the package floor preferably of a SnAg-based or SnSb-based solder. It surrounds at least part of the metallized portion of the fiber so as to hold the fiber in its desired position, in alignment with an optoelectronic device in the package. With Sn metallization on the fiber, this results in a highly reliable solder lock joint. A solder seal joint forms a hermetic seal between the nose tube and the Au metallized fiber. This solder is preferably 80Au/20Sn. A rigid cylindrical seal tube sleeve insert on the, fiber is designed to guide the fiber into the nose tube without bending or damaging the fiber.
Tower, et al., in U.S. Pat. No. 6,145,731, teach a method of making hermetic seals between optically transparent ceramic or glass member and a metallic housing. The member is then press-fit into the aperture, partially displacing metal from the walls of the aperture, forming an inner burr circumscribing the aperture. The walls of the aperture and the circumscribing burr are then coated with a second metal, preferably electroless nickel.